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Constant low-volume tone out of speakers

Hi, I have a home built system with a Gigabyte G31M-ES2L motherboard. It's running a Core 2 Duo E8400 with 4 GB of memory and an nVidia GeForce 8400 GS graphics adapter, and Windows 7 64-bit.

My problem is as follows: there is a constant low level audio tone coming out of both channels of the audio out jack. The volume of this tone is moderately low so it is not drowning out the normal audio but it is definitely noticeable (and annoying) when playing material with quiet passages. I haven't tried to measure its frequency exactly but I'm guessing it's in the 600-800 Hz range. It seems only to occur when Windows is running, i.e. I do not hear it during the BIOS startup and initial Windows boot. It is completely independent of any volume setting in Windows. The only way I have found to stop it is to disable the audio device in Device Manager. If I re-enable the driver, the sound begins again. I also tried disabling the "Azalia CODEC" in the BIOS. That got rid of the noise, but then I had no functionaling audio at all.

I also tried updating the motherboard drivers.

One other thing I've noticed is that the tone seems to occasionally chirp or warble slightly, and this seems to happen when any of the hard disks are acessed.

Anyone have any ideas as to what might be causing this and how to remedy it?

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This sounds like a low frequency harmonic is feeding through into your audio section, the original frequency is probably connected with data supply in and out of your hard drives. Your audio section should include filtering to keep these harmonics off the 'Audio out' so as not to interfere with your listening pleasure. A computer is a mass of different frequencies - everything from mains hum up to the CPU clock - this suggests that something has gone wrong with one of the filtering sections.

As this is probably non-repairable, unless you feel qualified to start trying to replace surface mounted components, then the only way to test is replacement. If you are using the on-board audio system, you could try to borrow a separate plug-in audio card to see if that stops the problem. If you are using a separate audio card, then try unplugging it and using the on-board sound system to see if that stops the noise. A consolation, if you decide a separate sound card is the way forward, is that they are not very expensive. My local computer store lists six models, with the most expensive being £UK 27.

If you are already using a sound card, one quick thing you could try first unplugging and re-seating it, and doing the same for its power connector.

Thank you for those ideas - I think I will give a separate audio card a try.

Today I noticed something that may in fact be related. If you stand a bit away from the display, you can just barely make out some very subtle horizontal bars which seem to sporadically float from bottom to top. These look a lot like what I used to call "hum bars", but again they are very subtle. I don't know what the vertical refresh rate of a typical LCD monitor is but I wonder if this is related...

What you might want to check is where your CD/DVD audio cables are routed from the mainboard to your optical drive. Those wires are usually shielded with an outer wire braid that prevents harmonics from entering the audio circuits, however, some instances do happen where the placement of this wire do pick up oscillation from other near-by circuits.

So what I would try first is relocating your wiring a little bit and see if there is a difference in these stray harmonics frequency or complete elimination of them.

Edited to add:

Electrical hardware located close to the computer can cause harmonics in audio circuits. Florescent lighting is one such example. Telephone modem wires can contain stray harmonics when users have DSL. Cable modems placed close by to computer may also cause stray harmonics.

Powerful RF sources such as AM radio station transmitters, FM radio station transmitters and even a powerful television station transmitter.